A bicycle is normally provided with a rear derailleur active on a cogset, which consists of a series of coaxial toothed wheels (sprockets) having different diameters and numbers of teeth coupled with the hub of the rear wheel.
A bicycle can also be provided with a front derailleur associated with a chainset, which consists of a series of toothed wheels (crown gears) having different diameters and numbers of teeth associated with a pin of the bottom bracket driven in rotation by a pair of pedals.
Usually, there are two or three crown gears of the chainset having increasing diameters starting from the crown gear closest to the frame of the bicycle.
The derailleur, whether front or rear, engages a transmission chain extending in a closed loop between the cogset and the chainset, moving it on toothed wheels having a different diameter and number of teeth, so as to obtain different gear ratios.
In particular, downward gearshifting is said when the chain passes from a toothed wheel of larger diameter to a toothed wheel of smaller diameter, and upward gearshifting is said when the chain moves from a toothed wheel of smaller diameter to a toothed wheel of larger diameter. Concerning this, it should be noted that with reference to the front derailleur, downward gearshifting corresponds to the passage to a lower gear ratio and upward gearshifting corresponds to the passage to a higher gear ratio.
The movement in the two directions of a derailleur is obtained through an actuation device mounted on the handlebars so as to be easily maneuvered by the cyclist.
More specifically, in a mechanical gearshift, the front derailleur is moved between the crown gears of the chainset, in a first direction (upwards) by a traction action exerted by an inextensible cable that is normally sheathed (commonly called Bowden cable), in a second opposite direction (downwards) by the release of the traction of the cable and by the elastic return action of a spring foreseen in the derailleur itself.
The front derailleur is actuated by a linkage that actuates a chain guide adapted to physically move the chain between different engagement positions on the crown gears.
The chain guide comprises an inner plate and an outer plate substantially parallel to one another and substantially parallel to the planes on which the crown gears lie. The inner plate is spaced from the outer plate and both are simultaneously and rigidly translated by the traction or by the release of the control cable.
During downward gearshifting, the outer plate of the derailleur intercepts the chain and moves it from the crown gear engaged making it fall onto a smaller crown gear arranged closer to the frame.
During upward gearshifting, the inner plate of the derailleur intercepts the chain and moves it, accompanying it, towards a larger crown gear arranged further from the frame.
A pair of mechanical end stops limits the excursion of the derailleur to prevent it from being able to move the chain too far towards the frame or too far away from the frame (thus ensuring that the chain does not move beyond the crown gears).
In the actuation device, the control cable is actuated in traction or release through winding and unwinding on a rotor element, commonly called cable-winding bush, the rotation of which is actuated by the cyclist with a suitable control lever, or with two control levers (a first lever for upward gearshifting and a second lever for downward gearshifting).
In any case, the actuation device must foresee for the cable-winding bush to be held still in rotation in a number of predetermined angular positions, angularly spaced apart by predetermined indexing angles. This function is obtained with so-called indexers, many types of which are known in the prior art, variously active between the cable-winding bush and the fixed casing of the device.
One type of indexer used to control the front derailleur comprises an indexing bush and a so-called ball-carrying disc, the interaction of which determines the stable obtaining of the predetermined angular positions of the cable-winding bush.
In particular the ball-carrying disc and the indexing bush are fitted on a control shaft set in rotation by the control lever actuated by the cyclist. The ball-carrying disc and the cable-winding bush are constrained in rotation to the control shaft, whereas the indexing bush does not rotate with respect to the control shaft.
On the indexing bush there are two or more stop areas (as a function of the number of crown gears) that engage in respective stable equilibrium positions a ball carried by the ball-carrying disc. Usually, the stop areas are arranged on a circular sector of the indexing bush and are aligned circumferentially on it, in other words they are arranged the same radial distance from the center of the indexing bush. The stop areas are angularly spaced apart by angles coinciding with the indexing angles of the cable-winding bush.
In order to perform upward gearshifting, the cyclist actuates the relative control lever and the control shaft is set in rotation making the ball of the ball-carrying disc carry out an angular rotation with respect to the indexing bush until it reaches a further stop area in which it stops. During such a rotation, the ball-carrying disc and indexing bush move apart, in contrast to the action of a cup spring, through the effect of the axial thrust exerted by the ball that passes over the stop area occupied to occupy a circumferentially next one. Such a rotation of the ball with respect to the indexing bush corresponds to a rotation of the ball-carrying disc and, therefore, of the cable-winding bush. The winding of the control cable on the cable-winding bush determines the translation of the chain guide plates of the derailleur that derail the chain on a crown gear having immediately greater diameter allowing gearshifting.
Similarly, in order to perform downward gearshifting, the cyclist actuates the relative control lever and the control shaft is set in rotation taking the ball of the ball-carrying disc into the stop area immediately preceding in which it stops. Such a rotation of the ball-carrying disc corresponds to a rotation of the cable-winding bush that unwinds the control cable. The release of the control cable determines a translation of the chain guide plates of the derailleur that derail the chain onto the crown gear having a smaller diameter.
Usually, the indexing bush comprises more than one stop area to position the front derailleur at the smallest crown gear. For example, it is possible to foresee three stop areas and in particular a first stop area corresponding to a position of the front derailleur substantially against the lower end stop (in which the control cable has the least pull or is even loosened), a second stop position in which the front derailleur is slightly distanced from the lower end stop and perfectly centered on the smallest crown gear and a third position in which the front derailleur is practically aligned, by the inner plate, with the smallest crown gear.
These three stop areas allow the chain to engage any of the toothed wheels of the rear cogset without it sliding against one of the chain guide plates of the front derailleur. Indeed, it should be noted that the rear cogset has an excursion, in the direction of the rotation axis thereof, of a few centimeters, making the chain more or less askew as a function of the toothed wheel of the cogset engaged.
The cyclist, therefore, actuates the control lever to engage the most appropriate stop area as a function of the toothed wheel of the cogset engaged by the chain when it is on the smallest crown gear of the chainset.
During downward gearshifting from the largest crown gear, the three stop areas do not impede gearshifting, since the chain is pushed by the outer plate of the front derailleur so as to carry out a jump in substantial free fall towards the smallest crown gear of the chainset. Therefore, the third of the aforementioned stop areas can also be sufficient for the outer plate of the derailleur to push the chain by a sufficient amount to make it fall onto the smaller crown gear.
On the other hand, upward gearshifting requires that the inner plate accompanies the chain until it engages the largest toothed wheel.
For this reason, usually, at the positioning of the chain on the largest crown gear there is a single stop area on the indexing bush of the front derailleur, such as to ensure that the derailleur has an effective excursion to engage the chain on the largest crown gear.